Nuclear spin polarized 3He gas is being applied for a variety of research experiments in physics as documented for instance by the following publications:                J. Becker, H. G. Andresen, J. R. M. Annand, K. Aulenbacher, K. Beuchel, J. Blume-Werry, Th. Dombo, P. Drescher, M. Ebert, D. Eyl, A. Frey, P. Grabmayr, T. Groβmann, P. Hartmann, T. Hehl, W. Heil, C. Herberg, J. Hoffmann, J. D. Kellie, F. Klein, K. Livingston, M. Leduc, M. Meyerhoff, H. Möller, Ch. Nachtigall, A. Natter, M. Ostrick, E. W. Otten, R. O. Owens, S. Plützer, E. Reichert, D. Rohe, M. Schäfer, H. Schmieden, R. Sprengard, M. Steigerwald, K.-H. Steffens, R. Surkau, Th. Walcher, R. Watson and E. Wilms; “Determination of the Neutron Electric Form Factor from the Reaction 3He(e; e‘n’) at Medium Momentum Transfer”; Eur. Phys. J. A 6, (1999) 329-344        D. Rohe, P. Bartsch, D. Baumann, J. Becker, J. Bermuth, K. Bohinc, R. Böhm, S. Buttazzoni, T. Caprano, N. Clawiter, A. Deninger, S. Derber, M. Ding, M. Distler, A. Ebbes, M. Ebert, I. Ewald, J. Friedrich, J. M. Friedrich, R. Geiges, T. Groβmann, M. Hauger, W. Heil, A. Honegger, P. Jennewein, J. Jourdan, M. Kabrau, A. Klein, M. Kohl, K. W. Krygier, G. Kubon, A. Liesenfeld, H. Merkel, K. Merle, P. Merle, M. Mühlbauer, U. Müller, R. Neuhausen, E. W. Otten, Th. Petitjean, Th. Pospischil, M. Potokar, G. Rosner, H. Schmieden, I. Sick, S. {hacek over (S)}irca, R. Surkau, A. Wagner, Th. Walcher, G. Warren, M. Weis, H. Woehrle, M. Zeier; “Measurement of the neutron electric form factor Gen at 0.67 (GeV/c)2 via ”, Phys. Rev. Lett. 83 (1999) 4257        W. Heil, J. Dreyer, D. Hofmann, H. Humblot, E.-Levievre-Berna, F. Tasset; “3He neutron spin-filter”; Physica B 267-268 (1999) 328-335).        
In magnetic resonance tomography (MRT) for medical applications also spin polarized nuclei other than the usual protons, namely, helium-3 (3He) and xenon-129 (129Xe), are being discussed nowadays. These spin polarized gases are suited in particular for investigating the ventilation of the lung. This is being described for instance in the following publications:                P. Bachert, L. R. Schad, M. Bock, M. V. Knopp, M. Ebert, T. Groβmann, W. Heil, D. Hofmann, R. Surkau, E. W. Otten; “Nuclear Magnetic Resonance Imaging of Airways in Humans with Use of Hyperpolarized 3He ”; MRM 36 (1996) 192-196;        M. Ebert, T. Groβmann, W. Heil, E. W. Otten, R. Surkau, M. Leduc, P. Bachert, M. V. Knopp, L. R. Schad, M. Thelen; “Nuclear magnetic resonance imaging with hyperpolarized 3He”; THE LANCET, 347 (1996) 1297-1299;        H.-U. Kauczor, R. Surkau, T. Roberts; “MRI using hyperpolarized noble gases”; Eur. Radiol. (8) (1998) 820-827,        J. P. Mugler, B. Driehuys, J. R. Brookeman, G. D. Cates, S. S. Berr, R. G. Bryant, T. M. Daniel, E. E. de Lange, J. H. Downs Jr, C. J. Erickson, W. Happer, D. P. Hinton, N. F. Kassel, T. Maier, C. D. Phillips, B. T. Saam, K. L. Sauer, M. E. Wagshul; “MR Imaging and Spectroscopy Using Hyperpolarized 129Xe Gas: Preliminary Human Results”; MRM 37 (1997) 809-815.as well as in the patent applications:        WO9527438A1 of 4.4.1995, “Magnetic Resonance Imaging using hyperpolarized Noble Gases” and        WO9737239A1 of 28.2.97, “Enhancement of NMR and MRI in the presence of hyperpolarized Noble Gases”.        
The term “Polarization degree” (P) indicates that fraction of atoms, the nuclear spins (I) of which are oriented together with their magnetic moments (μI) along the direction of an external magnetic field (B). In order to enable MRT on a gaseous species, one requires B to be 4 to 5 orders of magnitude higher than PBoltzmann which is the degree of polarization the gas attains in relaxed thermal equilibrium. In an external magnetic field of value B of the experimental set-up PBoltzmann is connected to the Boltzmann constant k, the absolute temperature T and the magnetic dipole energy—μIB by the eq.PBoltzmann=tan h(μIBT/kT)  (1)  (1)
Highest magnetic fields are being used in medical MR scanners, but still PBoltzmann is <<1 and hence can be calculated in good approximation as PBoltzmann=μIB/kT. Typical values in scanners are B=1.5 T and T=300 K. Consequently, a 3He polarization would attain only a value of PBoltzmann 3.9.10−6, whereas values P≦1.10−2 are necessary in order to compensate the loss of signal which is due to the much lower atomic density in the gaseous phase as compared to tissue. Gases with such high polarization degrees are termed also “hyperpolarized”. Their preparation is performed by known procedures, preferably by optical pumping (OP). In spite of the high polarization degree one has to provide relatively large quantities of hyperpolarized gas for the applications; for instance a quantity of order 0.5 to 11 is necessary per patient for studying lung ventilation.
The quality of the gas as tracer substance in MRT is measured by its polarization degree because the contrast of an MRT-image increases linearly with the polarization degree. At each excitation of nuclear resonance, the polarization vector is being flipped out of the axis of the magnetic field by a so-called flip angle (α). Through each of these excitations, the actual value of the hyperpolarization P is being irreversibly reduced to the value P.cos α. Aspiring a given image contrast, the flip angle may hence be chosen the smaller, the higher the value of P is in order to keep the product P.sin α which determines the signal strength constant. A higher polarization enables therefore a larger number of excitations with one and the same bolus of tracer gas. This offers the possibility to increase per tracer bolus either the spatial resolution or the number of images acquired at spatial resolution or the number of images acquired at constant spatial resolution. The latter is particular important for functional lung studies for which totally new sources of information are being opened by the application of hyperpolarized 3He gas. Such methods are described for instance in the publications:                W. G. Schreiber, K. Markstaller, B. Eberle, H.-U. Kauczor, N. Weiler, R. Surkau, G. Hanisch, M. Thelen; “Ultrafast MR-Imaging of Lung Ventilation Using Hyperpolarized Helium-3”; Eur. Radiol. 9, (1999) B28,        A. J. Deninger, B. Eberle, M. Ebert, T. Groβmann, W. Heil, H.-U. Kauczor, L. Lauer, K. Markstaller, E. Otten, J. Schmiedeskamp, W. Schreiber, R. Surkau, M. Thelen, N. Weiler; “Quantification of regional intrapulmonary oxygen partial pressure evolution during apnea by 3He MRI”; im Druck in J. Magn. Res. (November 1999),        X. J. Chen, H. E. Müller, M. S. Chawla, G. P. Cofer, B. Driehuys, L. W. Hedlund, G. A. Johnson; “Spatially Resolved Measurements of Hyperpolarized Gas Properties in the Lung in vivo. Part I: Diffusion Coefficient”; MRM, in press (June 1999).        
On the other hand, one may also reduce at higher P the amount of tracer gas necessary for reaching a given image contrast in order to minimize the alteration of normal lung ventilation, according to viscosity and diffusivity of ordinary air, by the admixture of 3He. The smaller the tracer bolus, the more realistic and valuable are the data end results. In producing nuclear spin polarized gases, one has to pay attention, therefore, not only to the yield, but in particular also to a high polarization degree.
There is a problem, however, to achieve high polarization degrees, for instance P>30%. Here the method of optical pumping of an alkaline vapor (mostly rubidium) followed by a spin polarization exchange between polarized alkali atoms and unpolarized 3He atoms may be applied. Such procedures and applications are described in the following publications:                B. Driehuys, G. D. Cates, E. Miron, K. Sauer, D. K. Walter and W. Happer; “High-volume production of laser-polarized 129Xe”; Appl. Phys. Lett. 69:12 (1996) 1668-1670,        Thad G. Walker, William Happer; “Spin-exchange optical pumping of noble-gas nuclei”; Review of Modern Physics 69:2 (1997) 629-642,        K. C. Hasson, P. L. Bogorad, B. Driehuys, G. Kameya, B. Wheeler, and D. Zollinger; “Polarized Helium-3 Production and Transportation System”; Eur. Radiol. 9 (1999) B16, further descriptions can be found in        WO9640585A1 vom 19.12. 1996: “Method and System for producing polarized 129Xe Gas”,        WO9639912A1 vom 7.6. 1996: “Coatings For Production Of Hyperpolarized Noble Gases”,        U.S. Pat. No. 5,545,396 vom 8.13. 1996: “Magnetic resonance imaging using hyperpolarized noble gases”,        U.S. Pat. No. 5,557,199 vom 17.9. 1996: “Magnetic resonance monitor”,        U.S. Pat. No. 5,612,103 vom 18.3. 1997: “Coatings for production of hyperpolarized noble gases”,        U.S. Pat. No. 5,617,860 vom 8.4. 1997: “Method and system for producing polarized 129 Xe gas”,        U.S. Pat. No. 5,642,625 vom 1.7. 1997: “High volume hyperpolarizer for spin-polarized noble gas”,        U.S. Pat. No. 5,789,921 vom 4.8. 1998: “Magnetic resonance imaging using hyperpolarized noble gases”,        U.S. Pat. No. 5,785,953 vom 28.7. 1998: “Magnetic resonance imaging using hyperpolarized noble gases”,        U.S. Pat. No. 5,809,801 vom 22.9. 1998: “Cryogenic accumulator for spin-polarized xenon-129”,        U.S. Pat. No. 5,860,295 vom 19.1. 1999: “Cryogenic accumulator for spin-polarized xenon-129”.        
The most powerful apparatus of this kind, is described in Hasson et al (1999) (s. above); it achieves a 3He yield of e.g. 2 bar l/h at P=10% or 0.38 bar l/h at P=30% or 0.11 bar l/h at P=40%.
An alternative procedure, the direct optical pumping of 3He in a low pressure discharge followed by compression, achieves high polarization, at present for instance P>30%, and simultaneously high yields, e.g. 0.5 bar l/h (equivalent to 12 bar l/d). For that 3He gas is taken from a reservoir and nuclear spin polarized by absorption of circularly polarized laser light at λ=1083.2 nm within a gas discharge at a pressure of 1 mb. Compressed thereafter and stored, the polarized gas is at disposal for various purposes in fundamental physics research as well as for medical applications. This procedure is described in detail in the following publications:                G. Eckert, W. Heil, M. Meyerhoff, E. W. Otten, R. Surkau, M. Werner, M. Leduc, P. J. Nacher, L. D. Schearer; “A dense polarized 3He target based on compression of optically pumped gas”; Nucl. Inst. & Meth. A 320 (1992) 53-65        J. Becker, W. Heil, B. Krug, M. Leduc, M. Meyerhoff, P. J. Nacher, E. W. Otten, Th. Prokscha, L. D. Schearer, R. Surkau; “Study of mechanical compression of spin-polarized 3He gas”; Nuc. Instr. & Meth. A (346) (1994) 45-51        W. Heil, H. Humblot, E. W. Otten, M. Schäfer, R. Surkau, M. Leduc; “Very long nuclear Relaxation times of spin polarized helium 3 in metal coated cells”; Phys. Lett. A 201 (1995) 337-343        R. Surkau; “Entwicklung und Test eines 3He-Neutronen-Spinfilters”; Dissertation an der Johannes Gutenberg-Universität Mainz (1995)        R. Surkau, J. Becker, M. Ebert, T. Groβmann, W. Heil, D. Hofmann, H. Humblot, M. Leduc, E. W. Otten, D. Rohe, K. Siemensmeyer, M. Steiner, F. Tasset, N. Trautmann; “Realization of a broad band neutron spin filter with compressed, polarized 3He gas”; Nuc. Instr. & Meth. A 384 (1997) 444-450        W. Heil, H. Humblot, E. W. Otten, M. Schäfer, R. Surkau, M. Leduc; “Very long nuclear Relaxation times of spin polarized helium 3 in metal coated cells”; Phys. Lett. A 201 (1995)337-343        R. Surkau; “Entwicklung und Test eines 3He-Neutronen-Spinfilters”; Dissertation an der Johannes Gutenberg-Universität Mainz (1995)        R. Surkau, J. Becker, M. Ebert, T. Groβmann, W. Heil, D. Hofmann, H. Humblot, M. Leduc, E. W. Otten, D. Rohe, K. Siemensmeyer, M. Steiner, F. Tasset, N. Trautmann; “Realization of a broad band neutron spin filter with compressed, polarized 3He gas”; Nuc. Instr. & Meth. A 384 (1997) 444-450.        
The state of the art is described in the publications Surkau (1995), Surkau et al (1997) and                R. Surkau, A. J. Deninger, J. Bermuth, M. Ebert, T. Groβmann, W. Heil, L. Lauer, E. Otten, J. Schmiedeskamp; “Highly polarized 3He for Lung MRI”; Eur. Radiol. 9 (1999) B15.        
The state of the art polarizer comprises five sections:                1) Gas is delivered from a gas delivery unit        2) The nuclear spins are then polarized at pressures between 0.5 mb and 5 mb in a polarization unit        3) A compression unit then compresses the gas to a final pressure of up to 10 bar        4) A storage unit stores the polarized gas for an extended period of time for the purpose of transport and application.        5) The units which contain polarized gas have to be embedded in a homogenous magnetic field, a prerequisite for sustaining the polarization over extended periods of time. A typical field strength of 0.8 mT is being applied for instance.        
That magnetic field has to fulfill a certain limit of homogeneity. Let the relative, transverse field gradient Gr fulfill for example the limitGr=∂Br/∂r/B0<5*10−4/cm.  (2)
Then one achieves at pressures of e.g. 200 mb or more which occur e.g. in storing gases still a gradient induced longitudinal relaxation time of T1G>47 h. Here one has used the relationT1=(p/Gr2*17000)h/(bar cm2),  (3)given in the publication:                L. D. Schearer, G. K. Walters; “Nuclear Spin-Lattice Relation in the Presence of Magnetic-Field Gradients”; Phys. Rev. 139:5A (1965) 1398-1402        G. D. Cates, S. R. Schaefer, and W. Happer; “Relaxation of spins due to field inhomogenities in gaseous samples at low magnetic fields and low pressures”; Phys. Rev. A 37 (1989) 2877-2885.        
In order to achieve within the relevant region the necessary homogeneity, required above for preserving the polarization, the magnetic field is formed by five circular coils of equal diameter with ampere turns number and relative distances to each other suitably adapted. The two outer coils on each side are being arranged cylindrically with respect to the central coil. All five coils are connected in series in order to maintain a uniform current in au of them. The correct relative ampere turns number is then adjusted for each coil by choosing the suitable turns number. This procedure has the advantage that the relative contribution of each coil to the total field remains constant even in case of small current drifts. Hence, a homogenous field is formed reliably within the volume of the coil arrangement.
The gas delivery assembly comprises a 3He reservoir and a sluice which takes periodically gas from the reservoir and delivers it to a purification stage. This way, a 3He flux is formed which is being purified from contaminant gases ex other noble gases. Here, one uses highly porous getter material ST 707, supplied by SAS, Milan. The first stage, at a temperature of about 250° C., absorbs predominantly molecules by cracking them to atoms and binding these atoms. In a second stage at room temperature, the getter material diminishes the par pressure of hydrogen to very low values, i. e. hydrogen is adsorbed in the getter effectively. Thereafter, the purified gas is guided through a capillary and a liquid nitrogen trap into the assembly for optical pumping. The capillary establishes by means of a pressure gradient a continuous flow into the polarizing unit. On the other hand, it prevents a back diffusion of polarized gas into the purification unit and into regions outside of the homogenous field, where the polarization would be destroyed quickly.
The polarization unit comprises four cylindrical, 1 m long and 75 mm wide cells (320). In these cells, high frequency fields power a gas discharge at pressures between 0.1 and 3 mb and produce metastable 3He gas (3He*). This 3He* absorbs resonant light at μ=1083.2 nm from a suitable light source (for instance, cw Nd:LMA solid state laser). This light is being circularly polarized by polarization optics and irradiated parallel to the external magnetic field. The angular momentum which is transferred by light absorption to the atoms is orienting the nuclear spins parallel or antiparallel to the magnetic field axis depending whether the light polarization is right or left handed. For details on the physical processes see the relevant literature e.g.:                F. D. Colegrove, L. D. Schearer, K. Walters; “Polarization of 3He Gas by Optical Pumping; Phys. Rev. (132) (1963) 2561-2572,        P. J. Nacher and M. Leduc; “Optical pumping in 3He with a laser”; J. Phys. (Paris) 46 (1985) 2057-2073.        G. Eckert, W. Heil, M. Meyerhoff, E. W. Otten, R. Surkau, M. Werner, M. Leduc, P. J. Nacher, L. D. Schearer, “A dense polarized 3He target based on compression of optically pumped gas”; Nucl. Inst. & Meth. A 320 (1992) 53-65.        
After passing a cell once, the laser beam is reflected by a dichroitic mirror, thus giving the light a second chance for absorption. Fluorescence light from the discharge at λ=668 nm is transmitted, however, by the mirror. The degree of circular polarizaton of this fluorescence light is being measured and transformed via pressure dependent gauge factors into the a absolute degree of nuclear polarization by means of a polarizaton monitor which is described in the publications:                N. P. Bigelow, P. J. Nacher, M. Leduc; “Accurate optical measurement of nuclear polarization in optically pumped 3He gas”; J. Phys. II France 2 (1992) 2159-2179        W. Lorenzon, T. R. Gentile, H. Gao, R. D. McKeown; “NMR calibration of optical measurement of nuclear polarization in 3He”; Phys. Rev. A 47:1 (1993) 468-479.        
More details are given below. The cells are connected in series. So the gas is flowing slowly through the cells and gaining steadily polarization.
After passing another liquid nitrogen trap, the gas enters periodically e.g. every ten seconds via an inlet valve into the compression volume of the first piston compressor. It is then compressed by a forward move of the piston and pushed via an outlet valve into a storage cell. The piston is driven by a linear drive, powered by compressed air. The lead through of the tappet to the interior of the compressor which meets ultra high vacuum standards is tightened by a bellow welded to the rear flange of the cylinder on one side and to rear end of the tappet on the other. All components of the compressor are made out of titanium or bronze in order to minimize polarization losses by wall contact with magnetic material. Said metals meet also ultra high vacuum standards and are wear and tear resist. The piston has a diameter of 140 mm and a stoke of 110 mm; it is tightened and guided by double lip O-rings. The intermediate storage is usually filled up to 200 mb before a second, in principal identical compressor compresses the gas to final pressures between 1 and 10 bar. The second compressor has a piston diameter of 60 mm and a stroke of 94 mm; it empties the intermediate storage by 6 consecutive compression cycles with a period of also 10 s.
The gas, highly compressed by the second compressor stage is pushed out into the storage cell, flanged to the compressor. This storage cell an be closed by its own valve and taken off the polarizer for transport. Such transportable cells are also used for long storage of the polled gas. An exemplary design for medical application is being built from a suitable, ironfree and diffusion resistant Supremax glass from the company Schott Glass, Mainz. In such cells, relaxation times of up to 100 h can be achieved. Moreover, the relaxation time may be prolonged further by help of a suitable coating of the inner surface. Up to 200 h relaxation time have been achieved that way, being described in the publications:                W. Heil, H. Humblot, E. W. Otten, M. Schäfer, R. Surkau, M. Leduc; “Very long nuclear relaxation times of spin polarized helium 3 in metal coated cells”; Phys. Lett. A 201 (1995) 337-343        W. Heil, J. Dreyer, D. Hofmann, H. Humblot, E.-Levievre-Berna, F. Tasset; “3He neutron spin-filter”; Physica B 267-268 (1999) 328-335.        
The method of optical pumping of 3He in a low pressure discharge followed by compression has been well investigated theoretically. Yet it is still a great and many-sided physical and technical challenge to derive from these principles devices and to develop them to maturity such that the requirements regarding yield and polarization degree as well as reliability and guarantee of quality are met. Production, storage and subsequent transport of the hyperpolarized gas to the client requires innovative procedures and devices. Therefore, hyperpolarized gas was produced hitherto on a small scale only. Construction and operation of an apparatus for efficient production of highly polarized 3He gas demanded expertise in various fields of atomic and laser physics, in nuclear magnetic resonance, in ultra-high vacuum techniques, in handling special materials like Titanium or glass, for instance. Particularly in the case of clinical applications many different applicants at different places have to be supplied with gas of guaranteed quality.
Quality management can be realized best if the gas is produced in central facilities by specialists with powerful apparatuses. The polarized gas is then stored and delivered to the client.